Plasma arc torches are used widely in the processing (e.g., cutting and marking) of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. The plasma gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
FIG. 1 illustrates a known starting sequence that is used to obtain a transferred arc for the purposes of plasma arc cutting. A pilot arc is first generated between the electrode (cathode) and the nozzle (anode) (step 10). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch, or any of a variety of contact starting methods. Next, a gas flow passes through the nozzle exit orifice (step 12) causing the pilot arc to attach to the nozzle end face near the nozzle exit orifice.
Transfer height is defined as the maximum distance that can be maintained between the end of the torch and the workpiece to accomplish successful transfer of the arc from the nozzle to the workpiece. Transfer height generally is a function of the pilot current and the pilot arc relay opening threshold current level. For example, increasing the pilot current or lowering the relay opening threshold (i.e., the current that opens the relay) increases the transfer height. An increased transfer height generally improves the ease of operation of the torch.
When spaced from a workpiece a distance that exceeds the maximum transfer height, the torch remains in the pilot arc mode. However, once the torch is brought to within the maximum transfer height (step 14), ionized gas reduces the electrical resistance between the electrode and the workpiece forming a transferred arc between the electrode and the workpiece (step 16).
The torch sustains the two arcs (i.e., the pilot arc and transferred arc) due to current sharing between the nozzle and the workpiece. When current sharing exists, the power source output current equals the current level of the transferred arc plus the current level of the pilot arc. The current flow to the workpiece is sensed to determine when there is sufficient current flow to satisfy a predetermined threshold value capable of reliably sustaining a transferred arc (step 18). When this occurs, the nozzle is electrically disconnected from the starting circuit by opening a relay (step 20), extinguishing the pilot arc while maintaining the transferred arc between the electrode and the workpiece. Once the arc is transferred to the workpiece, the current to the torch is adjusted to a cutting current level (step 22). The torch is operated in this transferred plasma arc mode, characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting or marking of the workpiece.
In some applications, such as hand cutting and expanded metal cutting using a pilot arc controller, the torch can operate in pilot arc mode for a significant fraction of the power supply duty cycle. During these applications, pilot arc wear on the nozzle can become significant. This pilot arc wear reduces nozzle life and degrades the performance of the torch.
Experiments have shown that nozzle wear is a function of pilot current, i.e., nozzle wear increases with increasing pilot arc current. One method for improving arc transfer without increasing pilot arc current excessively involves decreasing the threshold current level of the pilot arc relay. However, the threshold level must be maintained at a high enough value to assure stable arc transfer. In presently available plasma arc torch systems, it has proven difficult to provide a pilot arc current level that is low enough to reduce nozzle wear, yet high enough to provide reliable transfer of the arc to the workpiece at a reasonable transfer height.